1. Field of the Disclosure
This disclosure herein relates to the field of electrical resistivity well logging tools and methods.
2. Description of the Related Art
Wellbores or boreholes for producing hydrocarbons (such as oil and gas) are drilled using a drill string that includes a tubing made up of jointed tubulars or a continuous coiled tubing with a drilling assembly, also referred to as the bottom hole assembly (BHA), attached to its bottom end. The BHA includes a number of sensors, formation evaluation tools, and directional drilling tools. A drill bit attached to the BHA is rotated with a drilling motor in the BHA and/or by rotating the drill string to drill the wellbore. One of the formation evaluation tools commonly used is an electromagnetic wave propagation logging tool for the determination of electrical properties of formations surrounding a borehole. Such tools are generally referred to in the oil and gas industry as the resistivity tools. These logging tools make measurements of apparent resistivity (or conductivity) of the formation that, when interpreted, provide information about the petrophysical properties of the formation surrounding the borehole and fluids contained therein. Resistivity logging tools also are commonly used for logging wells after the wells have been drilled. Depending upon the application, these tools are referred to as the wireline resistivity tools, logging-while-drilling (LWD) or measurement-while-drilling (MWD) resistivity tools. These tools also are often referred to as induction logging tools. For the purpose of this disclosure, the term resistivity tool or induction logging tool is meant to include all such versions.
A typical resistivity tool includes a number of receiver coils or antennas placed at various axial distances from one or more transmitter coils or antennas. Alternating current is passed through the transmitter coil, which induces alternating electromagnetic fields in the earth formations. Voltages are induced in the receiver coils as a result of electromagnetic induction phenomena related to the alternating electromagnetic fields induced in the formation surrounding the wellbore.
Resistivity tools typically employ co-axial antennas. These antennas do not provide information relating to anisotropy at low relative dipping angles and direction of an approaching bed relative to the drill bit. To obtain such information, axially-spaced multi-component antennas are used, which spacing results in extremely long tools. Also, measurements made by each such antenna requires depth interpolation for its location for processing and interpretation of the measured signal.
The disclosure herein provides improved apparatus and methods that address some of the above-noted issues relating to resistivity tools.